The present invention relates generally to a technology of purifying an exhaust gas of a lean-burn internal combustion engine capable of burning an air-fuel mixture in an oxygen excessive state and, more particularly a technology of purifying the exhaust gas of the lean-burn internal combustion engine having a nitrogen oxide occluding/reducing catalyst disposed in an exhaust system.
In the field of the internal combustion engine mounted in an automobile and the like, for reducing a quantity of fuel burned, there has been increasingly developed the lean-burn internal combustion engine capable of burning the air-fuel mixture in which an air-fuel ratio is higher than a theoretical air/fuel ratio (which means an oxygen excessive state). What is known as this type of lean-burn internal combustion engine is a so-called intake port injection type lean-burn internal combustion engine including an intake port formed to generate a tumble flow or a swirl flow of the air-fuel mixture flowing into a combustion chamber, and a fuel injection valve so attached that an injection port thereof faces the intake port.
With the intake port injection type lean-burn internal combustion engine, the fuel is injected out of the fuel injection valve at the latter stage of an exhaust stroke through the early stage of an intake stroke, and is uniformly mixed with fresh air at the intake port, thus flowing into the combustion chamber. On this occasion, the air-fuel mixture forms the tumble flow or the swirl flow. Then, when the air-fuel mixture is ignited by a spark plug, flames in the vicinity of the spark plug diffuse over within the combustion chamber along the tumble flow or the swirl flow, and the combustion of the air-fuel mixture in the lean state is speeded up.
In the intake port injection type lean-burn internal combustion engine, the air-fuel mixture with the fuel and the fresh air being substantially uniformly mixed with each other, is introduced into the combustion chamber. Therefore, as a fuel concentration is made much leaner by reducing the fuel injection quantity, the fuel concentration in the vicinity of the spark plug becomes leaner, with the result that the ignition by the spark plug becomes impossible.
By contrast, there has been increasingly developed a cylinder injection type lean-burn internal combustion engine having the fuel injection valve so attached that the injection port thereof faces the combustion chamber. In the cylinder injection type internal combustion engine, the fresh air is introduced into the combustion chamber by the intake stroke, and subsequently the fuel is injected from the fuel injection valve by a compression stroke, thereby forming the air-fuel mixture combustible only in the vicinity of the spark plug. At this time, there is formed a combustible air-fuel mixture layer in the vicinity of the spark plug in the combustion chamber of the internal combustion engine, and air layers are formed in other regions, whereby a so-called stratified state occurs. The thus stratified air-fuel mixture is burned, wherein the combustible air-fuel mixture in the vicinity of the spark plug serves as an ignition source.
Accordingly, the cylinder injection type lean-burn internal combustion engine is capable of making the fuel concentration within the entire combustion chamber leaner than by the intake port injection type lean-burn internal combustion engine, and providing both reduction of fuel consumption and the stable combustion state.
On the other hand, the exhaust system of the internal combustion engine is provided with a ternary catalyst for purifying HC, CO and NO.sub.x in the exhaust gas. The ternary catalyst is constructed to efficiently oxidize HC and CO when the air/fuel ratio of the exhaust gas falls within a predetermined range of the theoretical air/fuel ratio, and efficiently reduces NO.sub.x. Hence, when the lean-burn is carried out in the above-described lean-burn internal combustion engine, an oxygen concentration in the exhaust gas increases, and the air/fuel ratio of the exhaust gas increases higher than the predetermined range described above. Then, the ternary catalyst, while it can oxidize HC and CO, is incapable of reducing NO.sub.x sufficiently.
Such being the case, the nitrogen oxide occluding/reducing catalyst is disposed in the exhaust system of the lean-burn internal combustion engine. The nitrogen oxide occluding/reducing catalyst has such characteristics as to occlude nitrogen oxide (NO.sub.x) present in the exhaust gas when in a so-called lean state where the oxygen concentration of the exhaust gas flowing in is high, and to desorb the occluded nitrogen oxide (NO.sub.x) by making the nitrogen oxide (NO.sub.x) react to carbon monoxide (CO) and hydro carbon (HC) in the exhaust gas and reducing it into nitrogen (N.sub.2) when the oxygen concentration of the exhaust gas flowing in decreases while the hydro carbon (HC) increases.
In the lean-burn internal combustion engine having the nitrogen oxide occluding/reducing catalyst, the nitrogen oxide occluding/reducing catalyst absorbs the nitrogen oxide (NO.sub.x) contained in the exhaust gas when in the lean-burn process, and reduction components (carbon monoxide (CO) and hydro carbon (HC)) in the exhaust gas are increased before the nitrogen oxide (NO.sub.x) absorption quantity of the nitrogen oxide occluding/reducing catalyst is saturated, thus effecting a so-called rich spike, then, it is required that the exhaust gas be thereby purified on the catalyst by desorbing the nitrogen oxide (NO.sub.x) occluded by the nitrogen oxide occluding/reducing catalyst.
As an apparatus for efficiently desorbing and purifying the nitrogen oxide (NO.sub.x) occluded by the nitrogen oxide occluding/reducing catalyst, there is known an exhaust gas purifying apparatus of an internal combustion engine which is disclosed in Japanese Patent Application Laid-Open Publication No.6-173660.
This prior art exhaust gas purifying apparatus of the internal combustion engine is so designed, in the intake port injection type lean-burn internal combustion engine, to desorb and purify the nitrogen oxide (NO.sub.x) occluded by the nitrogen oxide occluding/reducing catalyst by injecting from the fuel injection valve the same quantity of fuel as when forming the air-fuel mixture in the oxygen excessive state, and, at the same time, introducing the gas containing a vapor fuel generated in a fuel tank into an exhaust passageway disposed upstream of the nitrogen oxide occluding/reducing catalyst and into an intake system of the internal combustion engine, and thereby to increase hydro carbon (HC) in the exhaust gas flowing into the nitrogen oxide occluding/reducing catalyst.
The exhaust gas purifying apparatus described above does not take into consideration a quantity and a concentration of the vapor fuel, or a time required for the vapor fuel actually arrives at the nitrogen oxide occluding/reducing catalyst from the time of starting a supply of the vapor fuel. Thus, this exhaust gas purifying apparatus is not only incapable of supplying the nitrogen oxide occluding/reducing catalyst with the exhaust gas containing a desired quantity of reduction components, but also incapable of supplying the nitrogen oxide occluding/reducing catalyst with the exhaust gas containing the reduction components at a desired timing. As a result, the nitrogen oxide (NO.sub.x) occluded by the nitrogen oxide occluding/reducing catalyst is not sufficiently desorbed and purified, and the nitrogen oxide occluding/reducing catalyst becomes the saturated state, with the result that the nitrogen oxide (NO.sub.x) is released into the atmospheric air without being purified and an exhaust emission might be worsened.
In the case of applying the exhaust gas purifying apparatus described above to the cylinder injection type lean-burn internal combustion engine, especially when the vapor fuel is supplied during the stratified combustion process, the interior of the combustion chamber cannot be made in the stratified sate. This might cause possibilities that the combustion becomes unstable, the fuel concentration in the vicinity of the spark plug becomes higher than needed, which causes failure of ignition by the spark plug, and thus, results in an accidental fire.